The mechanism of chloroquine resistance in Plasmodium falciparum is not clearly understood and remains very controversial. Mutations in Plasmodium falciparum pfmdrl and pfcrt genes have been associated with chloroquine resistance. However, there is no absolute correlation between mutations in each of these two genes and in vitro or in vivo chloroquine resistance. This fact implies the potential involvement of other determinants of chloroquine resistance in the parasite. This research is primarily directed at investigating the role of pfmdrl, pfcrt and additional determinants of chloroquine resistance in P. falciparum in Nigerian isolates. We also propose to use molecular markers of chloroquine and sulfadoxine-pyrimethamine resistance as a surveillance tool for monitoring changes in the efficacy of these drugs over time in Nigeria. Data generated by this research work will be made available to the national malaria control program in order to promote evidence-based policy on malaria therapy in Nigeria. To achieve these goals, the following activities will be carried out over a 3-year period. 1. Determine genetic correlates of chloroquine sensitivity in P. falciparum isolates from patients treated with chloroquine. The presence of mutations in P. falciparum chloroquine resistance determinant genes (i) pfcrt, and (ii) pfmdrl genes will be determined by DNA sequencing analysis. 2. Validate the role of chloroquine resistance determinants in P. falciparum clones prepared from patients' isolates. 3. Determine the background mutation prevalence in the DHFR and DHPS genes associated with P. falciparum sulfadoxine-pyrimethamine resistance in Nigerian isolates. 4.Establish a site in Nigeria that will serve as the national reference laboratory to study the development and spread of drug resistant Plasmodium falciparum over time in Nigeria. This research project provides a unique opportunity to use the study site in Nigeria as a front-line, in order to study and better understand the development and spread of drug resistance in the country. The research will be done primarily in Ibadan, at the Malaria Research laboratory, PIMRAT, University of Ibadan's College of Medicine in Collaboration with Dr. Christian Happi as an extension of the NIH grant 5 R01 GM61351-04; 05/01/2000-04/30/2004.